Conventional apparatus having DOI detectors include a DOI-PET (Positron Emission Tomography) apparatus, for example. In this apparatus, numerous DOI detectors first detect pair annihilation photons of 511 [keV] which are energy emitted from inside a patient administered with a positron-emitting radionuclide. When two DOI detectors detect photons within a definite period of time, they are counted as one pair of annihilation photons, and it is further determined that pair annihilation generating points exist on a straight line linking the DOI detector pair having detected them. RI (Radio Isotope) distribution images are created by accumulating such coincidence information and carrying out an image reconstruction process.
With the apparatus which calculates position information on the light source (=radiation source) from the position information on the DOI detectors and creates medical images from this information as noted above, the quality of medical images is improved by acquiring detailed position information on the DOI detectors. In detecting positions in a planar direction, for example, there is a technique of subdividing detectors or using a multi-anode type photomultiplier tube (PMT) (see Nonpatent Documents 1 and 2, for example). For detection in a depth direction, there is a technique of using DOI detectors having multiple layers of photon detecting elements of different types (see Nonpatent Document 3, and Patent Documents 1-3, for example).    [Nonpatent Document 1]    http://www.nirs.go.jp/use/medical-imaging/ja/study/nextgeneration-pet2000/ninadama.html    [Nonpatent Document 2]    http://www.ricoh.co.jp/net-messensa/ACADEMIA/JAMIT/MITVM/PET/TANAKA04/text14.html    [Nonpatent Document 3]    A GSO depth of interaction detector for PET: IEEE Trans. Nucl. Sci., 45: 1078-1082, 1998.    [Patent Document 1]    Unexamined Patent Publication No. 2000-56023    [Patent Document 2]    Unexamined Patent Publication No. 2004-245592    [Patent Document 3]    Patent National Publication No. 2006-522925